February 16, 2011

Study helps explain how the brain sees motion

A test subject has the middle temporal
visual area (MT) of her brain temporarily
weakened so that researchers
can study how it affects her ability to
detect motion. Duje Tadin has found
that the MT area is responsible for
allowing humans to ignore background
motion in their environments and
instead focus in on more important
motion of smaller objects in the
foreground.

When elderly drivers get behind the wheel, they often confront the harrowing reality that they cannot easily see other cars, pedestrians, or cyclists moving around them. This frightening effect of aging, it turns out, is not necessarily a result of a reduced ability to perceive moving objects, as one might suspect, but a heightened awareness of the backdrop against which objects move.

A team of scientists led by Duje Tadin, an assistant professor of brain and cognitive sciences, has isolated the cause of the phenomenon, and the surprising results could not only help train elderly people to be better drivers, but they could also help psychiatrists better understand abnormal brain processes in psychological conditions like depression and schizophrenia. Their research, conducted at the Berenson-Allen Center for Noninvasive Brain Stimulation at the Harvard Medical School, was published in the Journal of Neuroscience.

In a healthy, young person, a brain region called the middle temporal visual area, or MT, actively suppresses often irrelevant background motion so that he or she can concentrate on the more important motions of smaller objects in the foreground. Previous studies have found that elderly people, as well as those with psychological conditions such as schizophrenia and depression, are better at perceiving motion in the background. But this above-average motion perception is not something to look forward to as we age. Because the brain is spending its limited resources paying attention to the unimportant motions of background objects, it has a harder time noticing the motions of smaller objects.

“The amount of visual information around us is huge, and we don’t have the brain power to process it all,” Tadin says. “Evolutionarily speaking, moving objects are the most important visual features to detect quickly, because they could be your lunch or they could want to eat you for lunch. It just makes sense that our vision prioritizes processing them.”

Tadin and his colleagues discovered that the MT was responsible for this effect by using a technique called transcranial magnetic stimulation. By precisely placing magnetic coils on the back of a subject’s head, the scientists stimulated the MT with electrical signals for 15 minutes to temporarily inhibit its functioning. Then, while the MT was less active, they tested how well subjects identified motions of smaller and larger objects on a computer screen.

They found that when the MT was inhibited, subjects had an easier time identifying the motion of large, background-like objects. These results indicate that an improperly functioning MT may be the cause behind better than normal perception of background motion in older adults.